Rōpū moni MySQL ma te whakamahi? Te whakamaarama taipitopito mo te roopu ma te tatau i roto i te paataka raraunga MySQL

MySQL Huihia te roopu ma te whakamahi?pātengi raraunga MySQLHe whakamaramatanga mo te roopu ma te tatau

MySQL GROUP BY tauākī

Ka whakarōpūhia e te tauākī GROUP BY te huinga hua i runga i te kotahi, neke atu rānei o ngā tīwae.

I runga i te rarangi whakarōpū ka taea e tatou te whakamahi i nga mahi COUNT, SUM, AVG, etc.

Rōpū MA te wetereo

SELECT column_name, function(column_name)
FROM table_name
WHERE column_name operator value
GROUP BY column_name;

Whakaaturanga tauira

Ko te tauira o tenei upoko e whakamahi ana i te hanganga ripanga me nga raraunga e whai ake nei.

SET NAMES utf8;
SET FOREIGN_KEY_CHECKS = 0;
-- ----------------------------
--  Table structure for `employee_tbl`
-- ----------------------------
DROP TABLE IF EXISTS `employee_tbl`;
CREATE TABLE `employee_tbl` (
  `id` int(11) NOT NULL,
  `name` char(10) NOT NULL DEFAULT '',
  `date` datetime NOT NULL,
  `singin` tinyint(4) NOT NULL DEFAULT '0' COMMENT '登录次数',
  PRIMARY KEY (`id`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8;
-- ----------------------------
--  Records of `employee_tbl`
-- ----------------------------
BEGIN;
INSERT INTO `employee_tbl` VALUES ('1', '小明', '2016-04-22 15:25:33', '1'), ('2', '小王', '2016-04-20 15:25:47', '3'), ('3', '小丽', '2016-04-19 15:26:02', '2'), ('4', '小王', '2016-04-07 15:26:14', '4'), ('5', '小明', '2016-04-11 15:26:40', '4'), ('6', '小明', '2016-04-04 15:26:54', '2');
COMMIT;

SET FOREIGN_KEY_CHECKS = 1;

Kia angitu te kawemai, mahia te korero SQL e whai ake nei:

mysql> set names utf8;
mysql> SELECT * FROM employee_tbl;
+----+--------+---------------------+--------+
| id | name   | date                | singin |
+----+--------+---------------------+--------+
|  1 | 小明 | 2016-04-22 15:25:33 |      1 |
|  2 | 小王 | 2016-04-20 15:25:47 |      3 |
|  3 | 小丽 | 2016-04-19 15:26:02 |      2 |
|  4 | 小王 | 2016-04-07 15:26:14 |      4 |
|  5 | 小明 | 2016-04-11 15:26:40 |      4 |
|  6 | 小明 | 2016-04-04 15:26:54 |      2 |
+----+--------+---------------------+--------+
6 rows in set (0.00 sec)

I muri mai ka whakamahia e matou te korero RUPAPA MA te whakarōpū i te ripanga raraunga ma te ingoa me te tatau e hia nga rekoata kei ia tangata:

mysql> SELECT name, COUNT(*) FROM   employee_tbl GROUP BY name;
+--------+----------+
| name   | COUNT(*) |
+--------+----------+
| 小丽 |        1 |
| 小明 |        3 |
| 小王 |        2 |
+--------+----------+
3 rows in set (0.01 sec)

Whakamahia me te ROLLUP

Ka taea e WITH ROLLUP te whakatinana i nga tatauranga rite (SUM, AVG, COUNT...) i runga i nga tatauranga kua whakarōpūhia.

Hei tauira, ka whakarōpūhia te ripanga raraunga i runga ake ma te ingoa, ka tatauhia te maha o nga wa i uru mai ai ia tangata:

mysql> SELECT name, SUM(singin) as singin_count FROM  employee_tbl GROUP BY name WITH ROLLUP;
+--------+--------------+
| name   | singin_count |
+--------+--------------+
| 小丽 |            2 |
| 小明 |            7 |
| 小王 |            7 |
| NULL   |           16 |
+--------+--------------+
4 rows in set (0.00 sec)

Ko te rekoata NULL e tohu ana i te maha o nga takiuru mo te katoa.

Ka taea e tatou te whakamahi coalesce ki te tautuhi ingoa hei whakakapi i te NUll, coalesce syntax:

select coalesce(a,b,c);

Whakaahuatanga tawhā: ki te a==null, whiriwhiria b; ki te b==null, kowhiria c; ki te a!=null, kowhiria a; ki te kore abc, whakahokia kore (kore tikanga).

I te tauira e whai ake nei mena kei te putua te ingoa ka whakamahia e matou te tapeke:

mysql> SELECT coalesce(name, '总数'), SUM(singin) as singin_count FROM  employee_tbl GROUP BY name WITH ROLLUP;
+--------------------------+--------------+
| coalesce(name, '总数') | singin_count |
+--------------------------+--------------+
| 小丽                   |            2 |
| 小明                   |            7 |
| 小王                   |            7 |
| 总数                   |           16 |
+--------------------------+--------------+
4 rows in set (0.01 sec)